Continuous reflux refining of metals

ABSTRACT

A method is provided for the continuous purification of an impure metal in a column consisting of a slurry of metal crystals in liquid metal, said column having a relatively hot zone and a continuous temperature gradient between said zones; the temperature of the cool zone being such as to permit coexistence of liquid and crystalline metal, and said hot zone being at a temperature sufficient to melt the crystals. A portion of the column in the said cool zone is chilled to form a coherent mass of crystals of the metal; then incipient melting in the mass is allowed to occur to release crystals from the mass into the column of the crystals in the liquid metal in the cool zone of said column. The crystals are transported through the temperature gradient and into the hot zone, wherein melting of the crystals occurs and the hot zone thereby becomes enriched with the pure liquid metal. 
     In normal operation, impure feed metal is supplied to the column; purified liquid metal is withdrawn from the hot zone; and impurity-enriched metal is withdrawn from the cool zone.

This is a division, of application Ser. No. 616,816 filed Sept. 25, 1975and now U.S. Pat. No. 4,043,802.

This invention relates to a method and apparatus for the continuousrefining of metals or other crystalline substances. Although the greaterpart of the following description will relate to metal refining, it willbe appreciated that application of the principles disclosed herein tothe purification and refining of other crystalline substances is withinthe scope of the invention described.

The prior art (see Arnold et al. U.S. Pat. No. 2,540,977 Re 24038,2,540,083; McKay et al. in "Chemical Engineering Progress 62", 104-112,(1966) and elsewhere) shows the use of a reflux refining for thepurification of low-melting organic substances wherein a slurry of thecrystalline substance produced by preliminary chilling of the liquidsubstance is fed into the top or central portion of a column which isheated at the base and has provision for removal of the purified liquidsubstance from the base of the column and for recycling the impurityenriched liquid from the top of the column.

Such a process is not directly applicable to the purification of metalsor other relatively high-melting materials. One approach to this problemis a process for refining metals, organic compounds and other substanceswhich comprises cooling the top of a column of the liquid substance toform crystals of the substance and allowing the crystals to fall througha temperature gradient to the base of the column where they areconverted to liquid by heating. Part of the liquid formed was acceptedas final product, while the remainder (reflux) was displaced upwards bythe descending crystals, thereby to cause the impurities present inthese to pass into the reflux liquid, and also, by a washing action, todisplace and absorb the impurities associated by entrainment in, andattachment to, the crystals. Such a process, referred to as "refluxrefining", has three main criteria which must be met for satisfactoryoperation: Firstly, the crystals are made in the cold region andtransported to the hot region at which almost all of the heat input intothe column is supplied. Secondly, a greater weight of crystals is formedthan that of the feed added, in order to allow for the use of liquidreflux; the extent of the reflux is equal to the weight of crystals madeless the amount of pure product taken from the hot region. Thirdly, thecomposition of the alloy to be refined must be hypoeutectic with respectto the pure metal being sought.

A further necessary condition for successful reflux refining is that thecolumn must be maintained full of a slurry of crystals at all timesduring operation.

We also described specific methods for forming the crystals, whichinvolved chilling the liquid medium by contact with cooled fluid or acooled surface of a rigid body to which the crystals were non-adherent.In each case continuous production of the crystals was envisaged.

In practice it has proved difficult to find suitable fluids which willallow operation of our previously described process in metal refining.Difficulties have also been encountered in finding suitable solidsurfaces to which non-adherence of metal crystals can be reliablyachieved.

We have also found that there are substantial advantages to be gained bymodifying the techniques of crystal production which we have previouslydescribed. In particular, we have now found that metal crystals of size,shape and composition suitable for reflux refining can best be producedby partial melting of a previously formed mass of the crystals which hasbeen produced by chilling a part of the liquid contained in the column.

According to the present invention, there is provided a method for thecontinuous purification of an impure metal which comprises providing acolumn consisting of a slurry of metal crystals in liquid metal, saidcolumn having a relatively cool zone and a relatively hot zone and acontinuous temperature gradient between said zones; the temperature ofthe cool zone being such as to permit coexistence of liquid andcrystalline metal, and said hot zone being at a temperature sufficientto melt the crystals; chilling a portion of the column in the said coolzone to form a coherent mass of crystals of the metal then allowingincipient melting in the mass to occur to release crystals from the massinto the column of the crystals in the liquid metal in the cool zone ofsaid column; transporting the crystals through the temperature gradientand into the hot zone, wherein melting of the crystals occurs and thehot zone thereby becomes enriched with the pure liquid metal.

For most practical applications, the method also includes the steps ofsupplying impure feed metal (in liquid, slurry or solid forms asdescribed hereinafter) to the column and continuously withdrawing aportion of the liquid metal from the hot zone.

To attain the maximum benefits from the method of the invention, it isnecessary that the crystals be small in relation to the column diameter.For this reason it is highly desirable that the massive solid metalwhich is melted to form the crystals should consist of small crystals ofrelatively purer metal in a matrix of a relatively less pure lowermelting point material. Such a structure is exhibited by metals whichexhibit the phenomenon of micro-segregation during solidification.Microsegregation occurs whenever the first parts of the solid nucleiwhich form from the melt are of higher melting point than those partswhich form subsequently. The phenomenon is associated with the presenceof impurities which are sometimes present in crude metals beforerefining and will be well known to those skilled in the art. For examplethe presence of bismuth, tin and silver in lead bullion, and copper andsilicon in aluminium, give rise to a form of micro-segregation known asdendritic segregation in which the higher melting, purer metalcrystallizes in the form of dendrites with a "pine-tree" structure andthe lower melting, less pure material occupies the inter-dendriticvolumes.

Some impurities have a pronounced effect in "modifying" the size andshape of the dendritic crystals, and we have discovered that thesemodifying impurities can be used to produce, initially, a fine dendriticstructure which subsequently breaks down to yield a crystal of afavourable size and shape for enhanced purification by reflux refining.For example the addition of a trace of sodium to aluminium containingsilicon as an impurity, or the addition of approximately 0.5% tin or alesser proportion of silver to lead containing bismuth as impurity,produces a suitable modification of the dendritic structure whichfavourably affects the refining of these metals.

If the appropriate impurity or modifying metal or metals are not presentin the metal feed, it will be necessary to add the impurity metal ormetals to the feed or, less preferably, by adding the impurity metals tothe cool zone (usually near the top of the column) during the crystalforming operations.

According to one embodiment, the invention provides a method forcarrying out reflux refining of impure metals in a column which containsa slurry of suitable metal crystals and liquid metal, and which has atemperature gradient along its length, wherein crystals of suitablesize, shape and composition are generated within the column by chillingthe contents of the column near the cooler end of the column to form acoherent mass of crystals then allowing incipient melting in the mass tooccur to release crystals from the mass into the column; the crystalsare transported axially towards the hotter end of the column, therebycausing a displacement of liquid metal in the opposite direction to themovement of crystals; purified liquid metal is withdrawn from the hotterend of the column, while a smaller proportion of a liquid alloy enrichedin impurities is withdrawn from the cooler end of the column; the feedmetal being added at the cooler end or at an intermediate point in thecolumn.

Preferably, the crystals in the slurry are generated by partialremelting of the crystalline mass produced by chilling of a liquid metalcontaining impurities which give rise to dendritic segregation onfreezing to produce interdendritic volumes of a lower melting alloycomposition which melts during the partial remelting step to releasesolid crystals of slightly higher melting point into the column.

According to one aspect of the invention, the crystals are produced byintroducing into the cooler part of the column a massive body of thefeed metal, preferably in the form of an ingot. Such an ingot preferablyconsists of the feed metal and has a microsegregated structure asalready described. The cold metal body causes chilling of liquid in thecolumn in the vicinity of the body, thereby producing a coherent mass ofcrystals which adhere to the metal body. As the body becomes heated, bythe combined effects of heat transfer from the column and release oflatent heat of solidification which accompanies the freezing process,the crystal mass undergoes incipient melting, as described above, andthe crystals are released into the column.

Preferably, the solid feed metal is introduced in the form of discreteingots at intervals so that the introduction of each ingot causesproduction of crystals by chilling as already described and then theingot itself is in turn partially remelted to release further crystals,and to provide all or part of the feed metal to the column.

It is also within the scope of the invention to introduce the impurefeed metal in the form of a continuous ingot, whereby the stages ofcrystal formation, by chilling and partial remelting of the columnliquid, and subsequent partial remelting of the ingot occur successivelyas the ingot progresses into the column.

In a combination of these two approaches, a continuously cast ingot maybe introduced into the column in a stepwise fashion.

According to another aspect of the invention, the column liquid ischilled to provide crystals by an intrusive cooling element immersed inthe liquid column, and/or, less preferably, by cooling the externalsurface of liquid column, e.g., by a cooling jacket.

The cooling element or jacket is cooled intermittently to provideintermittent or "pulsed" chilling of the liquid to achieve the formationand subsequent incipient remelting of the crystal mass as alreadydescribed.

In this last embodiment of the invention, it will be clear that aseparate feed of liquid metal is required.

The invention also provides apparatus for carrying out the method justdescribed, said apparatus comprising vessel to contain a column ofliquid metal, heating means arranged so as to provide a relatively hotzone and a relatively cool zone in said column, means to first chill atleast a part of the liquid in the cool zone to form a coherent mass ofcrystals and then to allow incipient melting in the mass to releasecrystals from the mass to thereby form a slurry of the crystalline metalin the liquid metal, and means to withdraw liquid metal from the hotzone.

The vessel may be of constant cross section, or may be uniformly taperedto be narrowest in the vicinity of the cool zone and widest in thevicinity of the hot zone. More preferably, the vessel tapers outwardsmore rapidly in the vicinity of hot zone than elsewhere.

Preferably also, the heating means comprises intrusive heating elementsor a heated section of the vessel in the hot zone.

The apparatus may also include means for intermittently producing smallrelative motion of the crystals and the liquid in the column, bymovement of either the crystal mass or the liquid. This may be achieved,for example, by raising and lowering the level of the contents of thevessel, this procedure being referred to hereinafter as "pulsing".

The means for chilling the liquid may, as already indicated, compriseany suitable device which, at intervals, will introduce into the coolzone of the column preformed individual ingots of the feed metal. In thecrudest form of the apparatus, of course, the ingots may simply beinserted manually, but in more sophisticated arrangements, the apparatusmay include means for casting such ingots and/or suitable machinery forinserting the ingots into the column.

Alternatively, the chilling means may comprise means for introducing acontinuous ingot of the feed metal into the cool zone of the column,such introduction being either by continuous or stepwise movement of theingot. Thus, in this form, the apparatus may also include means forcasting the impure feed metal into a continuous ingot, and means forfeeding the ingot into the cool zone of the column. In this arrangementit will be convenient, therefore, to arrange the continuous castingapparatus and the liquid column in a linear arrangement.

In a further embodiment of the apparatus, the chilling means comprisesan intrusive cooling element which is partly or wholly immersed in thecolumn liquid in the cool zone. Such an element may conveniently be ofthe "cold finger" type, that is a hollow closed tubular body havingmeans for introducing a cooling fluid into the interior of the body andmeans for allowing the fluid to escape from the body after it hasperformed its cooling function. This and many other equivalentarrangements are well-known per se in the chemical and chemicalengineering arts.

Whichever chilling means is used, it is desirable that the chillingelement or ingot made to vibrate longitudinally i.e. in a directionparallel to the column and/or to have a rotating or circulatory motionabout the lengthwise axis of the column.

Further aspects and features of the method and apparatus of theinvention will now be described by reference to specific embodimentsdepicted in the accompanying diagrammatic drawings, in which:

FIG. 1 shows a simple laboratory form of the apparatus in which crystalformation is by manual addition of preformed ingots;

FIG. 2 shows an apparatus involving continuous ingot formation;

FIG. 3 shows a modification of the apparatus of FIG. 2, for productionof crystals by individual ingots;

FIG. 4 shows another modification of the apparatus of FIG. 2, forproduction of crystals by intrusive cooling elements; and

FIG. 5 shows the intrusive cooling element of FIG. 4 in detail.

The apparatus of FIG. 1 consists of a cylindrical refining vesselcomprising a stainless steel tube 1 arranged vertically and consistingof a refining section 2 and a melting section 3. Tube 1 is surrounded bya Pyrex glass tube 4 which is wound with an electrical heating element5. The vessel is encased in a thermal insulating layer 6.

The melting section 3 flares outwardly at its base and a heating element(not shown) is provided to heat the base 7 of the section 3. Outlets 8and 9 are provided at the top and bottom respectively of the vessel fortapping off the contents of the vessel as required. A layer of siliconeoil 10 floats on top of a liquid metal in the column to minimiseoxidation of the metal in the column.

Operation of the apparatus of FIG. 1 is described in Example 1hereinafter.

In FIG. 2, the part of the apparatus below the line A--A is common tothe apparatus as described in FIGS. 2, 3 and 4. It consists of acolumnar refining vessel 21, arranged vertically and consisting of arefining section 22 and a melting section 23. Both sections 22 and 23are of circular cross-section which increases towards the bottom of thesection, but the melting section 23 flares more rapidly than section 22.A heating element 26 is located in the base 24 of section 23 andintrudes into the interior of the section. As illustrated the section 23has outlet 27 for liquid metal.

The part of the apparatus above line A--A in FIG. 2 is associated withthe continuous ingot production and melting, and the withdrawal ofliquid from section 22 of vessel 21 for recycling. The apparatusconsists of a continuous ingot casting machine generally indicated at31, which may be of any suitable known type. The outlet conduit 32 ofthe casting machine 31 is coaxially connected to a perforated section 36which forms the upper part of the wall of the refining section 22 ofvessel 21. The perforated section 36 is surrounded by a heated gallery37 from which liquid metal may be recirculated if required to thecasting machine 31 (by means not shown).

In use the continuously cast ingot of metal from the machine 31 providesthe source of feed material for refining and simultaneously provides theforce necessary for pushing the ingot into the refining vessel 21. Thecold ingot of feed metal after leaving the continuous casting machine 31through conduit 32 passes into the top of the crystal slurry columncontained in the vessel 21. Here the cold metal chills the slurry and inso doing becomes cooled with a thick coherent layer of crystals. As theingot continues to move downwards towards the hotter zone of the column,the crystal layer sloughs off the ingot and disintegrates into a slurryof crystals. Still further down the column the ingot itself undergoespartial remelting and disintegration to release more crystals and liquidas feed to the column.

As it is desirable to produce an ingot which will readily disintegrateunder these latter conditions it is undesirable to produce homogenoussound billets in the casting machine. It is, in fact, preferred tooperate the casting machine in such a way that a defective ingot isproduced which contains many defects such as "layers", "cold-shuts" andthe like.

The crystals thus produced at the top of the refining section 22continue to move down through the section while reflux liquid isdisplaced upwards past the crystals. Ultimately the crystals, which havebeen purified by this reflux process are melted in section 23 and partof the now pure liquid metal is tapped off from the base of the section23. The impure reflux liquid discharges into the gallery 37.

The apparatuses of FIGS. 3 and 4 have a number of common integers whichare numbered accordingly. In FIGS. 3 and 4 the portion of the apparatusshown above the line A--A in FIG. 2 is replaced by an externally heatedtank 41 which is of larger diameter than the vessel 21 and in which theformation of crystals takes place. The level of the crystal/liquid metalslurry in the tank 41 is shown by the line 42. The tank 41 is providedwith an impeller type stirrer, two arms of which are shown at 43, whichis supported on a central shaft 44. The shaft 44 also supports a crossarm 45 from which (in FIG. 3) precast ingots 46 are supported by wires47 cast into the ingots. The shaft 44 is provided with means (not shown)to provide for rotation of the shaft and its attachments and also forvertical oscillation of the shaft.

In FIG. 4, instead of the ingots 46, the cross arm 45 supports a numberof intrusive cooling elements generally designated as 48 and shown indetail in FIG. 5. The intrusive elements 48 are provided with means(shown in FIG. 5) by which the elements can be intermittently chilledand then allowed to reheat.

In each instance the introduction of the cold ingots 46 or the chillingof the elements 48 results in the formation of a layer of crystals aspreviously described in relation to FIG. 2. The reheating of the ingots46 or elements 48 then results in sloughing off of the crystals with theformation of the required slurry. In the case of FIG. 3, the ingotsthemselves then subsequently become partially remelted and disintegratedto provide feed metal for the process.

In the case of FIG. 4, feed metal is added separately either to the tank41 (by means not shown) or else by injection of liquid feed metal, or apreformed slurry of crystals and liquid formed from the feed metal, intoan appropriate level of the vessel 21.

FIG. 5 shows a typical intrusive cooling element which has been usedexperimentally in the apparatus of FIG. 1 and which, with suitablemodifications as to scale, can be used in the apparatus of FIG. 4. InFIG. 5 the cooling element consists of a steel tube 51 which is closedas its bottom end 52. Tube 51 is provided with a side arm 53 and isclosed at the top by a resilient sealing plug 54. A part of the interiorof the tube 51 is insulated by a sleeve 55 of insulating material,preferably fused silica. A narrow bore tube 56 is inserted through theseal 54 and extends near to the bottom 52 of the tube 51. A very finetube 57 extends from the end of the tube 56 almost to the bottom 52 ofthe tube 51. By way of example, the tubes 56 and 57 can be coarse andfine hypodermic needles respectively.

The tube 56 is connected to a positive displacement metering pump which,at predetermined intervals, supplies water to the tube which emergesfrom the fine tube 57 in a fine spray and impinges on the inside of thetube 51. The walls of the tube are thereby cooled and formation ofcrystals occurs on the outer walls of the tube. Steam produced by theevaporation of the water emerges through the side arm 53. The insulatingsleeve 55 prevents excessive chilling of the tube 51 over that portionof the tube which passes through the surface of the slurry in tank 41.This results in favourable modification of the shape of the crystal masswhich forms around the cooling element.

In a typical arrangement, water is introduced into the cooling elementover a period of 20 seconds to 1 minute. Over the next 2 to 4 minutesthe crystal mass thus formed sloughs off the element and the cycle isthen repeated.

Each of the apparatus as described above has the capacity to produceall, or almost all, of the input metal as final product. In the case ofthe embodiments employing individual ingot feed, each ingot can producemore than its weight of crystals. For example, 1 ton of lead at roomtemperature has to have 13/4 times its latent heat of fusionrequirements in order to have its temperature raised to the meltingpoint, i.e. 1 ton of lead at room temperature can product 23/4 tons ofcrystals. Thus, for example, if 1 ton of impure lead is to be refined toyield 0.95 tons of pure metal (from the base of the vessel) and 0.05tons of impure liquid (from the top), the addition of lead at roomtemperature would enable up to 1.75 tons of reflux liquid to be used forpurification with the vessel. The extent of reflux liquid made and usedper ton of metal produced can be controlled in two ways. Firstly, bycontrolling the temperature of the solid below its melting point, i.e.the lower the temperature of the solid ingots added, the greater theamount of reflux liquid produced. Secondly, if the metal to be refinedexists in the liquid state, the extent of reflux can be controlled byfreezing only a fraction of the liquid as ingots, and adding the coldingots and the remaining liquid directly to the crystal forming tank.The higher the fraction of liquid cast as ingots, the higher the extentof reflux liquid formation. Consider, for example, the refining of oneton of impure liquid lead (which exists just above its melting point)into 0.95 tons of refined metal and 0.05 tons of impure metal. Suppose0.50 tons of the liquid is cast into ingots and cooled to roomtemperature (prior to crystal making) while the other 0.50 tons is addedas liquid to the crystal tank. The addition of the 0.50 tons of the coldingots to the column will produce 1.375 tons of crystals per ton of theoriginal liquid. Since the corresponding production of refined metal is0.95 tons, the amount of reflux liquid formed is 0.425 tons, and thereflux ration is 0.447.

Similar control can be exercised when using continuous ingot feed. Wherecrystal formation is carried out using an intrusive cooling element,control is even more readily achieved by simply varying the length ofthe cooling fluid injection period and/or the frequency of the fluidinjection.

A typical procedure for continuous reflux refining of metals using solidindividual ingots to produce crystals is described below, together withexamples of the results which can be obtained.

EXAMPLE 1

The apparatus of FIG. 1 was used. The overall height of the liquidcolumn was 154 cm and the diameter was 12.1 cm. The heater winding (5)was rated at approximately 200 watts and the base heater at about 1 kw(maximum).

The general method of operation for lead refining is as follows:

Prior to operation, the apparatus is filled with the alloy to berefined. The power inputs to the side heater (5) and the base heater areadjusted to melt the alloy to maintain the temperature of the alloy justabove the melting point of pure lead, i.e., about 340° C. Thisrepresents the "standby" condition of the apparatus. The silicone oillayer (10) is added.

Crystals are next made at the top by inserting cold feed ingots,previously cast with a wire insert and cooled to room temperature. Atypical ingot is of square cross section (2.5 cm side), 22 cm long, andweighs about 1.7 kg. The base heater is turned up slightly (from 350 to490 watts) while the column is filled as rapidly as possible withcrystals, while removing excess liquid through the top tap (8). Once thecolumn is full of crystals, the power input to the base heater isstepped up gradually to the final running setting, while operating astotal reflux, i.e. with no product being taken from the bottom tap (9).The starting up procedure takes about two hours, after which product istaken from the top and the base of the column in the ratio of about oneto nine. Running is continued while maintaining the column full ofcrystals at all times and the level of the silicone oil/lead surface ata near constant height.

Typical results achieved are shown in Table 1.

                                      TABLE 1                                     __________________________________________________________________________    Impure Lead Feed Composition: 1700 ppm Ag. 300 ppm Bi.                        Power                              Bottom                                     Input          Analyses (ppm)      production                                 Time  at base                                                                            Reflux                                                                            Top      Bottom     rate                                       (minutes)                                                                           (watts)                                                                            ratio                                                                             Ag   Bi  Ag     Bi  (kg/hr)                                    __________________________________________________________________________    0     490  Total        1340   196                                            30    490  "            54     95                                             70    730  "   10200                                                                              540 4.5    67                                             120   850  "            1.2    57                                             150   850  1.86         1.2    57  31                                         210   850  1.86         1.9    67  31                                         340   850  1.86                                                                              14700                                                                              890 0.83   87  31                                         __________________________________________________________________________

Using the apparatus described in Example 1 and employing similartechniques, with appropriate modifications as to temperatures and powerinputs the following results were obtained.

EXAMPLE 2

For an impure tin metal feed containing 1.36% lead and 0.06% bismuth,with a reflux ratio of 1.06, the lead and bismuth in the top stream were5.7% and 0.21% respectively and in the bottom stream, 0.040% and 0.006%respectively.

EXAMPLE 3

For the refining of zinc containing 0.1% lead and 0.05% cadmium, arefined product containing less than 1 ppm of lead and less than 5 ppmof cadmium was produced. The top stream contained 0.6% Pb and 0.3% Cd.

EXAMPLE 4

Impure aluminium containing as impurities mostly iron and siliconamounting to 0.2 percent in total was refined to yield purifiedaluminium containing 0.001 percent of silicon and iron. It was foundthat the addition of small amounts of sodium were effective in reducingthe size of the aluminium crystals formed, and enhancing the extent andspeed of the refining operation.

The claims defining the invention are as follows:
 1. Apparatus for thecontinuous purification of an impure metal containing metallicimpurities which are soluble in the solid metal, and which ishypoeutectic with respect to the pure metal sought, said apparatuscomprising:a vessel to contain a vertical column of a slurry ofcrystalline metal in liquid metal; heating means arranged so as toprovide a relatively hot zone and a relatively cool zone at oppositeends of said column; an intrusive cooling element means for partly orwholly immersing in the liquid metal in the cool zone and adapted toperiodically chill at least a part of the liquid in the cool zone toform a coherent mass of crystals and then to allow incipient melting inthe mass to release from the mass small crystals of relatively puremetal, thereby to form a slurry of the crystals in the liquid metal;means for causing the cooling element to vibrate longitudinally, in adirection parallel to the column and/or to have a rotating orcirculatory motion about the lengthwise axis of the column, thereby tofacilitate release of the crystals from the mass; means forintermittently producing small relative motion of the crystals and theliquid in the column thereby to further separate and disperse thecrystals released from the coherent mass and to facilitate transport bygravity of the crystals towards the hot zone; means to withdraw purifiedliquid metal from the hot zone and means to withdraw metal of increasedimpurity content from the cool zone; and means to supply the impuremetal in liquid form to the column at the cool zone and/or at a levelbetween the cool and hot zones.
 2. Apparatus as claimed in claim 1,wherein the cooling element is a hollow closed tubular body having meansfor introducing a cooling fluid into the interior of the body and meansfor allowing the fluid to escape from the body after it has performedits cooling function.
 3. Apparatus as claimed in claim 1, wherein thecooling element comprises a preformed ingot of the feed metal.